Method for manufacturing a semiconductor device

ABSTRACT

In a method for crystallizing an amorphous silicon film by a heat treatment that is effected for a duration of about 4 hours at about 550° C. using a catalyst element for accelerating the crystallization, the quantity of the catalyst element to be introduced into the amorphous silicon is precisely controlled. A resist mask  21  is formed on the surface of an amorphous silicon film  12  provided on a glass substrate  11 , and an aqueous solution  14 , e.g., an acetate solution, containing a catalyst element such as nickel at a concentration controlled in a range of from 10 to 200 ppm (need to be adjusted) is supplied dropwise thereto. After maintaining the state for a predetermined duration of time, the entire substrate is subjected to spin drying using a spinner  15 . A thin film of crystalline silicon is finally obtained by applying heat treatment at 550° C. for a duration of 4 hours.

BACKGROUND OF THE INVENTION

1. Industrial Field of Application

The present invention relates to a semiconductor device usingcrystalline semiconductor, and to a method for fabricating the same.

2. Discussion of Prior Art

Thin film transistors (referred to simply hereinafter as “TFTs”) arewell known as devices utilizing thin film semiconductors. The TFTs arefabricated by forming a thin film semiconductor on a substrate andprocessing the thin film semiconductor thereafter. The TFTs are widelyused in various types of integrated circuits, and are particularlynoticed in the field of switching elements that are provided to each ofthe pixels of active matrix liquid crystal display devices as well as indriver elements of the peripheral circuits thereof.

Amorphous silicon films can be utilized most readily as the thin filmsemiconductors for TFTS. However, an amorphous silicon film has aproblem that the electrical characteristics thereof are inferior. Thisproblem can be circumvented by using a thin film of crystalline silicon.Crystalline silicon film is also denoted as, for example,polycrystalline silicon, polysilicon and microcrystalline silicon. Athin film of crystalline silicon can be prepared by first forming a thinfilm of amorphous silicon, and then crystallizing it by heat treatment.

The heat treatment for the crystallization of the amorphous silicon filmrequires heating the film at a temperature of 600° C. or higher for aduration of 10 hours or longer. Such a heat treatment has a problem thata glass substrate cannot be used. For instance, a Corning 7059 glasscommonly used for the substrate of an active matrix liquid crystaldisplay device has a glass distortion point of 593° C., and is thereforenot suitable for large area substrates that are subjected to heating ata temperature of 600° C. or higher.

SUMMARY OF THE INVENTION

According to the study of the present inventors, it is found that thecrystallization of an amorphous silicon film can be effected by heatingthe film at 550° C. for a duration of about 4 hours. This can beaccomplished by first introducing a trace amount of nickel or palladium,or other elements such as lead, into the surface of the amorphoussilicon film.

The elements above (catalyst elements capable of accelerating thecrystallization of an amorphous silicon film) can be introduced into thesurface of the amorphous silicon film by plasma treatment or vapordeposition, or by ion implantation. The plasma treatment is a methodcomprising adding the catalyst elements onto the amorphous silicon filmby generating a plasma of an atmosphere such as gaseous nitrogen orgaseous hydrogen in a plasma CVD apparatus of a parallel plate type orof a positive column type, while using a material containing catalystelements as an electrode.

However, the presence of the catalyst elements in a large quantity inthe semiconductor is not preferred, because the use of suchsemiconductors greatly impairs the reliability and the electricstability of the device in which the semiconductor is used. That is, theelements such as nickel which accelerate the crystallization (catalystelements) are necessary in the crystallization of the amorphous siliconfilm, but are preferably not incorporated in the crystallized silicon.These objects can be accomplished by selecting an element which tends tobe inactive in crystalline silicon as the catalyst element, and byincorporating the catalyst element at a minimized amount for thecrystallization of the film. Accordingly, the quantity of the catalystelement to be incorporated in the film must be controlled with highprecision.

Also, in case of using nickel as the catalyst element, a crystallinesilicon film was fabricated from an amorphous silicon film by addingnickel by plasma treatment, and the crystallization process and the likewas studied in detail to obtain the following findings as a result:

(1) In case of incorporating nickel by plasma treatment into anamorphous silicon film, nickel is found to intrude into the film to aconsiderable depth of the amorphous silicon film before subjecting thefilm to heat treatment.

(2) The initial nucleation occurs from the surface from which nickel isincorporated.

(3) When a nickel layer is deposited on the amorphous silicon film byvapor deposition, the crystallization of an amorphous silicon filmoccurs in the same manner as in the case of effecting plasma treatment.

It can be concluded from the above findings that not all of nickel atomsincorporated by plasma treatment into the amorphous silicon filmfunction effectively, and that, more importantly, only a trace amount ofnickel need to be incorporated in the vicinity of the surface of theamorphous silicon film. Assumably, a point (or a plane) at which siliconis brought into contact with nickel contributes to the low temperaturecrystallization of amorphous silicon. Conclusively, nickel atoms arepreferably dispersed as finely as possible to accelerate thecrystallization reaction. In other words, “nickel atoms need to beintroduced in the vicinity of the surface of amorphous silicon film at aminimum concentration necessary for the low temperature crystallizationof the amorphous silicon film”.

A trace amount of nickel, i.e., a catalyst element capable ofaccelerating the crystallization of the amorphous silicon, can beincorporated in the vicinity of the surface of the amorphous siliconfilm by, for example, vapor deposition. However, vapor deposition isdisadvantageous concerning the controllability of the film, and istherefore not suitable for controlling precisely the amount of thecatalyst element that is incorporated in the amorphous silicon film.

In particular, the crystals can be grown in parallel with the plane ofthe silicon film from the region onto which the solution is applied tothe region onto which the solution is not applied. It is also confirmedthat this region of crystal growth contains the catalyst element at alow concentration and that it is extremely useful to utilize such acrystalline silicon film as an active layer region for a semiconductordevice. However, there remains a problem of how to selectively introducethe catalyst elements.

An object of the present invention is to provide a method forfabricating a thin film semiconductor of crystalline silicon,characterized in that it satisfies the following requirements:

-   -   (1) The catalyst element is introduced at a controlled and at a        minimum possible quantity;    -   (2) The catalyst element is introduced into selected portions;        and    -   (3) The process yields high productivity.

The present invention uses the following means to accomplish the objectabove. Specifically, a mask-patterned amorphous silicon film iscrystallized by bringing it into contact with either a pure catalystelement which accelerates the crystallization of the amorphous siliconfilm or a compound containing the catalyst element, while applying heattreatment thereto.

More specifically, a solution containing the catalyst element is appliedto the surface of an amorphous silicon film having a desired patternformed thereon using a resist. In this manner, the catalyst element isintroduced into the surface of the amorphous silicon film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a method of the present invention.

FIG. 2 shows a process of fabrication of an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is characterized in that the catalyst element isintroduced by bringing it into contact with the surface of the amorphoussilicon film having a pattern formed thereon by using a resist.

The present invention is also characterized in that the thin filmcrystalline silicon thus crystallized using the catalyst element isemployed in the constitution of an active region having at least one ofthe electric junctions such as a PN, a PI, and an NI junction of thesemiconductor device. Specifically mentioned as the semiconductordevices are a thin film transistor (TFT), a diode, and an opticalsensor.

The present invention are basically advantageous in the followingaspects:

-   -   (a) The concentration of the catalyst element in the solution        can be strictly controlled to a minimum and optimum value        suitable for increasing the crystallinity of the thin film        silicon;    -   (b) The quantity of the catalyst element introduced in the        amorphous silicon film can be controlled by adjusting the        concentration of the catalyst element in the solution so long as        the solution is brought into contact with the surface of the        amorphous silicon film;    -   (c) The catalyst element can be introduced into the amorphous        silicon film at an amount as low as possible, because the        catalyst element adsorbed by the surface of the amorphous        silicon film principally contributes to the crystallization of        silicon; and    -   (d) The catalyst element can be selectively introduced into the        surface of the amorphous silicon film by using a resist pattern;        thus, a semiconductor device utilizing the region crystallized        alone the transverse direction can be easily fabricated.

The solution containing a catalyst element for accelerating thecrystallization can be applied to the surface of the amorphous siliconfilm by using, for example, an aqueous solution or a solution based onan organic solvent. The “solution” as referred herein encompasses thosecontaining the catalyst element in the form of a compound dissolved inthe solution, and those containing the element in the form of adispersion. Preferably the kind of the solution is selected by takingthe affinity of the solvent with the catalyst element intoconsideration. It is also preferred to consider the contact angle of thesolution and the surface of the thin film in selecting the solution.When a fine pattern is formed, in particular, a material having a smallcontact angle is preferably used to process the amorphous silicon filmdeeply into the pattern.

The solvent containing the catalyst element may be selected from varioustypes of polar solvents such as water, an alcohol, an acid, or ammonia.

When nickel is used as the catalyst, it may be added in a polar solventin the form of a nickel compound. More specifically, it may be selectedfrom a group of representative nickel compounds, i.e., nickel bromide,nickel acetate, nickel oxalate, nickel carbonate, nickel chloride,nickel iodide, nickel nitrate, nickel, sulfate, nickel formate, nickelacetylacetonate, nickel 4-cyclohexylbutyrate, nickel oxide, and nickelhydroxide.

Otherwise, a non-polar solvent can be used in the solution containingthe catalyst element. For example, a solvent selected from benzene,toluene, xylene, carbon tetrachloride, chloroform, ether,trichloroethylene, and Freon can be used as well. It should be noted,however, that “polarity” is referred herein not in a strict senseconsidering dipole moment, but in a general sense based on chemicalcharacteristics.

In this case, nickel is incorporated in the solution in the form of anickel compound. Representative compounds to be mentioned include nickelacetylacetonate and nickel 2-ethylhexanoate.

It is also useful to add a surface active agent into the solutioncontaining the catalyst element. The surfactant increases the adhesionstrength of the solution and controls the adsorptivity. The surfactantmay be applied previously to the surface of the substrate onto which theamorphous silicon is deposited.

When metallic nickel is used as the catalyst, it may be dissolved intoan acid to provide a solution.

The description above is for a case nickel is dissolved completely in asolution. Nickel need not be completely dissolved in a solution, andother materials, such as an emulsion comprising metallic nickel or anickel compound in the form of a powder dispersed in a dispersant may beused as well.

The same as those mentioned in the foregoing applies to the case inwhich a catalyst element other than nickel is used.

When nickel is used as the catalyst element for accelerating thecrystallization of amorphous silicon, it may be incorporated in a polarsolvent such as water. However, on applying the solution to a thin filmof amorphous silicon directly, the solution is sometimes found to berepelled by the surface of the amorphous silicon. This can becircumvented by forming a thin oxide film 100 Å or less in thickness,and then applying a solution containing the catalyst element thereon. Inthis manner, a uniform coating can be formed on the surface of amorphoussilicon. It is also useful to improve the wettability of the amorphoussilicon with the solution by adding a surfactant and the like into thesolution.

A solution can be directly applied to the surface of an amorphoussilicon film by using a non-polar solvent such as toluene. For instance,a toluene solution of nickel 2-ethylhexanoate can be used favorably insuch a case. It is also effective in this case to previously apply anadhesive or a like material commonly used in the case of forming aresist coating. However, such an agent must be treated with care lest itshould be applied to an excessive amount, because the presence of theadditive in excess obstructs the addition of a catalyst element insideamorphous silicon.

The concentration of the catalyst element in the solution depends on thekind of the solution, however, roughly speaking, the concentration ofnickel by weight is from 1 ppm to 200 ppm, and preferably, from 1 ppm to50 ppm. The concentration is determined based on the nickelconcentration or the resistance against hydrofluoric acid of the filmupon completion of the crystallization.

The crystal growth can be controlled by applying the solution containingthe catalyst element to the selected portions of the amorphous siliconfilm. In particular, the crystals can be grown in parallel with theplane of the silicon film from the region onto which the solution isapplied to the region onto which the solution is not applied. The regionin which the crystals are grown in parallel with the plane of theamorphous silicon film is referred to as the region crystallized in thelateral direction.

It is also confirmed that this region crystallized in the lateraldirection contains the catalyst element at a low concentration. It isuseful to utilize a crystalline silicon film as an active layer regionfor a semiconductor device, however, in general, the concentration ofthe impurity in the active region is preferably as low as possible.Accordingly, the use of the region crystallized in the lateral directionfor the active layer region is useful in fabricating a device.

The use of nickel as the catalyst element is particularly effective inthe method according to the present invention. However, other usefulcatalyst elements can be used as well. Such catalyst elements includeNi, Pd, Pt, Cu, Aa, Au, In, Sn, Pd, Sn, Pd, P, As, and Sb. Otherwise,the catalyst element may be at least one selected from the elementsbelonging to the Group VIII, IIIB, IVb, and Vb of the periodic table.When iron (Fe) is selected as the catalyst element, an iron saltselected from compounds such as ferrous bromide (FeBr₂.6H₂O), ferricbromide (FeBr₃.6H₂O), ferric acetate (Fe(C₂)H₃O₂)₃.xH₂O), ferrouschloride (FeCl₂.4H₂O), ferric chloride (FeCl₃.6H₂O), ferric fluoride(FeF₃.3H₂O), ferric nitrate (Fe(NO₃)₃.9H₂O), ferrous phosphate(Fe(PO₄)₂.8H₂O), and ferric phosphate (FePO₄.2H₂O) can be used.

In case cobalt (Co) is used as the catalyst element, useful compoundsthereof include cobalt salts such as cobalt bromide (CoBr.6H₂O), cobaltacetate (Co(C₂H₃O₂)₂.4H₂O), cobalt chloride (CoCl₂.6H₂O), cobaltfluoride (CoF₂.xH₂O), and cobalt nitrate (Co(NO₃)₂.6H₂O).

A compound of ruthenium (Ru) can be used in the form of a rutheniumsalt, such as ruthenium chloride (RuCl₃.H₂O).

A rhodium (Rh) compound is also usable in the form of a rhodium salt,such as rhodium chloride (RhCl₃.3H₂O).

A palladium (Pd) compound is also useful in the form of a palladiumsalt, such as palladium chloride (PdCl₂.2H₂O).

In case osmium (Os) is selected as the catalyst element, useful osmiumcompounds are osmium salts such as osmium chloride (OsCl₃).

If iridium (Ir) is selected as the catalyst element, a compound selectedfrom iridium salts such as iridium trichloride (IrCl₃.3H₂O) and iridiumtetrachloride (IrCl₄) can be used.

In case platinum (Pt) is used as the catalyst element, a platinum saltsuch as platinic chloride (PtCl₄.5H₂O) can be used as the compound.

In case copper (Cu) is used as the catalyst element, a compound selectedfrom cupric acetate (Cu(CH₃COO)₂), cupric chloride (CuCl₂.2H₂O), andcupric nitrate (CU(NO₃)₂.3H₂O) can be used.

In using gold (Au) as the catalyst element, it is incorporated in theform of a compound selected from auric trichloride (AuCl₃.xH₂O), auric,chloride (AuHCl₄.4H₂O), and sodium auric tetrachloride (AuNaCl₄.2H₂O).

The catalyst elements can be incorporated not only by using a solutionsuch as an aqueous solution and an alcohol solution, but also by using asubstance selected from a wide variety of materials containing thecatalyst element. For instance, metal compounds and oxides containingthe catalyst element can be used as well.

EXAMPLES Example 1

The present example refers to a method which comprises forming a desiredmask pattern on the surface of an amorphous silicon film using a resistmask, and then introducing nickel into selected portions of theamorphous silicon film by applying a solution containing nickel to thesurface of the amorphous silicon film having thereon the mask pattern.

Referring to FIG. 1, the method according to the present example isdescribed below. First, a resist pattern 21 is formed as a mask on aglass substrate (a Corning 7059 class substrate, 100 mm×100 mm). Eithera positive or a negative resist can be used.

The resist mask 21 is patterned as desired by means of an ordinarypatterning process using photolithography. A thin silicon oxide film 20is deposited thereafter by irradiating an ultraviolet radiation ingaseous oxygen. The thin silicon oxide film 20 can be fabricated byirradiating the UV light for a duration of 5 minutes under gaseousoxygen. Assumably, a silicon oxide film 20 about 20 to 50 Å in thicknessis obtained in this step (FIG. 1(A)).

The ultrathin silicon oxide film 20 thus obtained is provided for an aimto improve the wettability of amorphous silicon film 12 on applyingthereto a nickel-containing solution in the later step. Instead ofirradiating a UV light, the oxide film can be formed by immersing thesubstrate into aqueous hydrogen peroxide heated to 70° C. for a durationof 5 minutes. A thermally oxidized film can be used as well.

After forming the oxide film, 5 ml (in case of a substrate 10×10 cm² inarea) of an aqueous acetate solution containing nickel at aconcentration of 100 ppm by weight is applied dropwise to the substrate.After spincoating the surface of the substrate by operating a spinner at50 rpm for a duration of 10 seconds to obtain a uniform aqueous coatingon the entire surface, the substrate is maintained as it is for aduration of 5 minutes. Spin drying at 2,000 rpm is effected for 60seconds thereafter. The retention of the aqueous coating on the surfaceof the substrate can be effected on a spinner, while rotating thesubstrate at a rate of 150 rpm or lower (FIG. 1(B)).

The resist mask 21 is removed thereafter by oxygen ashing to selectivelyform a region containing nickel adsorbed thereon. Instead of usingoxygen ashing, the resist mask can be removed by annealing it in oxygen.

The amorphous silicon film 12 is crystallized thereafter by applying aheat treatment at 550° C. (in gaseous nitrogen) for a duration of 4hours. It is found that the crystal growth occurs from the region 22into which nickel is incorporated, towards the region 23 in which nickelis not introduced. Referring to FIG. 1(C), nickel is directly introducedinto the region 24. Upon crystallizing the region 24, it can be seenthat the crystallization proceeds in the lateral direction to provide aregion 25. It is confirmed that crystals in the crystallized region 25grow approximately along the direction of the crystallographic <111>axis.

It is also useful to employ annealing after the step of crystallization.The annealing is effected by using a laser radiation or an intense lightequivalent thereto. A thin film of crystalline silicon further improvedin crystallinity can be obtained. A laser beam emitted from a KrFexcimer laser or a XeCl laser can be used. An infrared radiation is alsouseful for the annealing. Annealing can be effectively carried out byusing an infrared radiation, because infrared light is selectivelyabsorbed by silicon and not by the glass substrate.

By controlling the concentration of the solution and the duration ofretention of the solution on the surface of the amorphous silicon film,it is possible to control the concentration of nickel in the region ofdirect addition to a range of from 1×10¹⁶ atoms·cm⁻³ to 1×10¹⁹atoms·cm³. At the same time, the concentration of nickel in the regionof lateral crystal growth can be controlled to a range lower than thatof the region above.

The crystalline silicon film thus obtained according to the method ofthe present example is characterized in that it yields an excellentresistance against hydrofluoric acid. This is in clear contrast with thecase in which nickel is introduced by a plasma process, because inaccordance with the findings of the present inventors, thin filmcrystalline silicon obtained by crystallizing an amorphous silicon filmafter introducing nickel by a plasma process is inferior with respect tothe resistance against hydrofluoric acid.

For instance, the poor resistance against hydrofluoric acid is criticalin case of providing an electrode by perforating a silicon oxide filmdeposited as a gate dielectric or an interlayer insulating film on thesurface of the thin film crystalline silicon. In such a case, ingeneral, the silicon oxide film is removed by using a bufferedhydrofluoric acid. If a thin film crystalline silicon inferior inresistance against hydrofluoric acid is used, it is found extremelydifficult to remove the silicon oxide film alone without causing, damageto the thin film crystalline silicon.

However, if a thin film crystalline silicon having a sufficiently highresistance against hydrofluoric acid is used, the silicon oxide filmalone can be removed selectively by taking the advantage of the largedifference (selectivity ratio) between the etching rate of the siliconoxide film and the thin film crystalline silicon.

As described in the foregoing, the region of lateral crystal growthyields a high crystallinity, and yet, is very low in the concentrationof the catalyst element. Accordingly, the use of this region for theactive layer region is useful in fabricating a device. Morespecifically, the use of this region as a channel region of a thin filmtransistor is particularly useful.

Example 2

The present example relates to a case in which a catalyst element,nickel, is incorporated into a non-aqueous solvent, alcohol, and isapplied to the surface of an amorphous silicon film. In the presentcase, nickel is added into alcohol in the form of nickelacetylacetonate. The concentration of nickel is adjusted as desired. Theprocess steps thereafter are the same as those described in Example 1.

The present example is described in further detail below. First of all,nickel acetylacetonate is prepared for use as the starting material.Nickel acetylacetonate is soluble to alcohol, and decomposes at a lowtemperature. Accordingly, it can be readily decomposed by the heatduring the crystallization step.

Ethanol is used as the alcohol. Nickel acetylacetonate is added intoethanol at such a concentration that nickel should be present in thesolution at a concentration of 100 ppm.

The resulting solution is applied to the surface of an amorphous siliconfilm having already thereon a desired resist pattern formed by usingPhotonese. Photonese is used specifically in this case because it doesnot dissolve into alcohol after it is baked at 300° C. The amorphoussilicon film used in this case is a film 1,000 Å in thickness, which isdeposited by means of plasma CVD on a 100×100-mm² area glass substratehaving thereon a base silicon oxide film (2,000 Å in thickness).

A smaller amount of solution is necessary in this case as compared withthe case using an aqueous solution as in Example 1. This is ascribed tothe fact that the contact angle of alcohol is smaller than that ofwater. Thus, 2 ml of the solution is added dropwise to an area of100×100 mm².

The resulting state is retained for a duration of 5 minutes and driedthereafter using a spinner. Drying is effected by operating the spinnerat 1.500 rpm for a duration of 1 minute. Nickel salt is decomposed byheating the dried substrate at 350° C. for a duration of 60 minutes.Thus, in this manner, nickel as a catalyst element can be introducedinto the amorphous silicon film by allowing it to diffuse into amorphoussilicon film. The Photonese mask is removed thereafter by wet etchingusing hydrazine or by ashing. A crystalline silicon film can be obtainedin this manner by carrying out the crystallization process at 550° C.for a duration of 4 hours.

As a matter of course, similar to the case as described in Example 1,crystal growth occurs in the lateral direction from the region intowhich the catalyst element is introduced to the region into which nocatalyst element is added. Thus, a region of crystalline silicon isobtained extending in the lateral direction.

Example 3

The present example relates to a case in which nickel is introduced asthe catalyst element into selected portions of an amorphous silicon filmby forming an oxide film containing nickel on an amorphous silicon filmhaving thereon a resist pattern.

In the present example, an OCD solution containing a catalyst elementfor accelerating the crystallization is used to form an oxide filmcontaining the catalyst element on the amorphous silicon film, and theoxide film is crystallized thereafter by heating. The OCD solution asreferred herein is Ohka Diffusion Source manufactured by Tokyo OhkaKogyo Co., Ltd., and it comprises an organic solvent dissolved therein asilicon compound and additives. The OCD solution is useful, because asilicon oxide film can be readily obtained by applying the solution toan object and baking it thereafter. Furthermore, a silicon oxide filmcontaining impurities can be easily obtained by using this solution.

A Corning 7059 glass substrate 100 mm×100 mm in area is used in thepresent example.

An amorphous silicon film from 100 to 1,500 Å in thickness is depositedby plasma CVD or LPCVD. More specifically in this case, an amorphoussilicon film was deposited at a thickness of 1.000 Å.

The resulting substrate is subjected to a treatment using hydrofluoricacid to remove stains and natural oxide films, and a resist pattern isformed as desired. It should be noted that a resist material having asufficiently high resistance against the organic solvent in the OCDsolution is selected in this case.

An oxide film containing nickel as the catalyst element is formedthereafter. Referring to FIG. 1, the oxide film is formed in the mannerdescribed below on the portion indicated with numeral 14 correspondingto the solution referred in Example 1.

A solution containing 0.2% by weight Of SiO₂ and from 200 to 2,000 ppmof nickel is prepared by mixing an OCD solution, i.e., OCD Type 2 Si59000 manufactured by Tokyo Ohka Kogyo Co., Ltd., with a methyl acetatesolution containing dissolved therein nickel (II) acetylacetonate.

Then, 10 ml of the resulting solution is applied dropwise to the surfaceof amorphous silicon film. Spin coating is effected by operating aspinner at a rate of 2,000 rpm for a duration of 15 seconds. A siliconoxide film containing nickel is formed at a thickness of about 1,300 Åby effecting, prebaking at 150° C. for a duration of 30 minutes. Thetemperature of prebaking can be determined by taking the decompositiontemperature of the nickel compound into consideration.

The resist is removed thereafter using a stripping solution. Then, theresulting structure is subjected to heat treatment at 550° C. for aduration of 4 hours under gaseous nitrogen in a heating furnace. As aresult, a crystalline thin film of silicon can be obtained on thesubstrate. At the same time, crystal growth occurs in the lateraldirection from the region into which nickel is introduced towards theregion into which no nickel is added.

The heat treatment above can be carried out at a temperature of 450° C.or higher. If the heat treatment were to be effected at a lowertemperature, the treatment must be effected for a longer duration. Sucha long treatment unfavorably impairs the production efficiency. Ifheating at a temperature of 550° C. were to be carried out, on the otherhand, the problem of heat resistance of the glass substrate must beovercome.

The concentration of nickel in the OCD solution cannot be determinedalone, and is determined in correlation with the concentration of SiO₂in the solution. Furthermore, the concentration of nickel must bedetermined by taking other factors into consideration, because theamount of nickel which diffuses from the silicon oxide film obtainedfrom the OCD solution into the thin film crystalline silicon differsdepending on the temperature and the duration of heating.

Example 4

The present example relates to a method for fabricating an electronicdevice by using a region obtained by introducing nickel into selectedregions and then allowing crystal growth to occur in the lateraldirection (a direction in parallel with the surface of the substrate).The concentration of nickel in the active layer of the device can befurther lowered by employing the constitution according to the presentexample. This constitution is extremely favorable from the viewpoint ofelectric stability and reliability of the device.

Nickel can be incorporated by any of the methods described in theforegoing Examples 1 to 3.

The present example relates to a method for fabricating a TFT for use incontrolling pixels of an active matrix addressed device. Referring toFIG. 2, the method for fabricating the TFT according to the presentexample is described below. First, a substrate 201 is cleaned, and abase silicon oxide film 202 is deposited thereon at a thickness of 2,000Å by means of plasma CVD using gaseous TEOS (tetraethoxysilane) andoxygen as the starting materials. Then, an intrinsic (I-type) amorphoussilicon film 203 is deposited at a thickness of from 500 to 1,000 Å. Forinstance, an I-type amorphous silicon film 203 is formed at a thicknessof 1,000 Å in this case. A silicon oxide film 205 is formed thereafter.A region of exposed amorphous silicon is obtained in this manner.

Subsequently, a solution (specifically, an acetate solution) containing,nickel as the catalyst element for accelerating the crystallization isapplied according to the method described in Example 1. The nickelconcentration is 100 ppm. The details and the process steps are the sameas those described in Example 1. This step of coating may otherwise beeffected by either of the processes described in Examples 2 and 3.

The silicon film 203 is crystallized thereafter by effecting annealingat a temperature in the range of from 500 to 620° C., for example, at550° C., for a duration of 4 hours. The crystallization initiates fromthe region 206, i.e., a region in which the silicon film is brought intocontact with nickel, and proceeds along a direction in parallel with thesubstrate as indicated with an arrow in the figure. It can be seen inthe figure that region 204 is crystallized by directly introducingnickel, and that region 203 is crystallized in the lateral direction.The region 203 crystallized in the lateral direction is composed ofcrystals about 25 μm in size. Furthermore, it is confirmed that crystalsin the crystallized region 203 grow approximately along the direction ofthe crystallographic <111> axis (FIG. 2(A)).

The silicon oxide film 205 is removed thereafter. The oxide film formedon the surface of the region 206 is removed at the same time. Afterpatterning the silicon film 204, an island-like active layer region 208is formed by dry etching. Referring to FIG. 2(A), the region 206represents the region rich in nickel, because nickel is introduceddirectly therein. It is also confirmed that the front end of crystalgrowth contains nickel at a high concentration. Such regions containnickel in a concentration higher than those in the intermediate regions.Accordingly, the method according to the present example is designed assuch that the channel forming region in the active layer 208 does notoverlap those regions containing nickel at high concentration.

The surface of the active layer (silicon film) 208 is oxidized to form asilicon oxide film 209. The silicon oxide film 209 is obtained byallowing the active layer 208 to stand in an atmosphere containing 100%by volume of water vapor under a pressure of 10 atm and at a temperaturein the range of from 500 to 600° C., representatively, at 550° C. Thusis obtained the silicon oxide film 209 at a thickness of 1,000 Å. Bythus forming the silicon oxide film 209 by thermal oxidation, the entiresubstrate is maintained at 400° C. under 100% gaseous ammonia at 1 atm.

The silicon oxide film 209 is subjected to nitriding by irradiating aninfrared radiation having a peak intensity in a wavelength range of from0.6 to 4 μm, more preferably, for example, in a range of from 0.8 to 1.4μm for a duration of 30 to 180 seconds. The atmosphere under which theprocess is effected may contain from 0.1 to 10% of gaseous HCl.

Halogen lamp is used as the light source of the infrared radiation. Theintensity of the infrared radiation is controlled as such that thetemperature as monitored on a single crystal silicon wafer may fall in arange of from 900 to 1,200° C. More specifically, the temperature ismonitored on the silicon wafer using a thermocouple buried in the wafer,and the detected value is fed back to the light source. In the presentexample, the temperature is elevated at a constant rate in a range offrom 50 to 200° C./sec, and is allowed to cool naturally at 20 to 100°C./sec. The method using infrared radiation is preferred, becauseinfrared radiation selectively heats the silicon film. Thus, the heateffect to the glass substrate is minimized (FIG. 2(B)).

Subsequently, an aluminum film is deposited at a thickness in a range offrom 3,000 to 8,000 Å by sputtering. For instance, a 6,000 Å thickaluminum (containing from 0.01 to 0.2% of scandium) is deposited. Thealuminum film thus obtained is patterned to obtain a gate electrode 210(FIG. 2(C)).

The surface of the thus obtained aluminum electrode is anodicallyoxidized thereafter to form an oxide layer 211 on the surface thereof.The process of anodic oxidation is effected in an ethylene glycolsolution containing from 1 to 5% of tartaric acid. Thus is obtained a2,000 Å thick oxide layer 211 on the surface of the aluminum electrode.Because the thickness of the oxide layer 211 thus obtained correspondsto the length of the offset gate region that is formed in the later stepof ion doping, the length of the offset gate region can be determined inthis step of anodic oxidation (FIG. 2(D)).

Then, by means of ion doping process (also known as plasma dopingprocess), an impurity (phosphorus) for rendering the portionN-conductive is added into the active layer region (which constitutessource/drain and a channel) in a self-aligned manner using the gateelectrode portion, i.e., the gate electrode 210 and the surroundingoxide layer 211. In the present example, phosphine (PH₃) is introducedas the doping gas to implant phosphorus at a dose in a range of from1×10¹⁵ to 8×10¹⁵ cm⁻², for example, at a dose of 4×10¹⁵ cm⁻² by applyingan accelerating voltage of from 60 to 90 kV, for example, at 80 kV.N-type impurity regions 212 and 213 are formed as a result. It can beseen from the figure that the impurity region is formed offset from thegate electrode for a distance of x. Such an offset structure iseffective, because the leak current (sometimes referred to as an “offcurrent”), which is observed when a reversed voltage (i.e., a negativevalue in case of an N-channel TFT) is applied to the gate electrode, canbe effectively lowered. Particularly in a TFT for use in the control ofa pixel electrode as in the present example, the leak current ispreferably as low as possible. By lowering the leak current, the chargecan be accumulated in the pixel electrode to reproduce favorable images.

Annealing is effected by irradiating a laser beam using a KrF excimerlaser (operating at a wavelength of 248 nm and a pulse width of 20nsec). The laser is operated to provide from 2 to 10 shots per site, forexample, 2 shots per site, at an energy density of from 200 to 400mJ/cm², for instance, at 250 mJ/cm². Furthermore, a more effectiveannealing can be realized by heating the substrate in a range of fromabout 200 to 450° C. (FIG. 2(E)).

Then, a 6,000 Å thick silicon oxide film 214 is deposited as aninterlayer insulating layer by means of plasma CVD. Furthermore, atransparent polyimide film 215 is formed thereon by spin coating toobtain a planarized surface. A 800 Å thick clear conductive film (an ITOfilm) is deposited by sputtering on the thus obtained planarizedsurface, and is patterned to provide a pixel electrode 216.

Contact holes are formed in the interlayer insulating layers 214 and215. Thus, electrode and interconnection 217 and 218 are formed by usingfilm comprising titanium nitride and aluminum. Finally, a pixel circuithaving a TFT for an active matrix device is obtained by annealing theresulting structure at 350° C. for a duration of 30 minutes undergaseous hydrogen at 1 atm.

As described above, the method according to the present inventionprovides a high performance semiconductor device with high productivityby using a crystalline silicon film which is obtained by a rapid and lowtemperature crystallization process. This rapid and low temperaturecrystallization is realized by selectively introducing a catalystelement using a resist.

In the example above, a layer containing a catalyst was formed on thesurface of the substrate by applying a solution containing the catalyst.However, it should be noted that it is also in the scope of the presentinvention a method which comprises forming previously a layer containingthe catalyst on the substrate, and then depositing thereon an amorphoussilicon film.

1. A method for manufacturing a semiconductor device comprising: forminga semiconductor layer over a substrate; forming a gate insulating filmover the semiconductor layer; forming a gate electrode over the gateinsulating film; forming a first insulating film over the semiconductorlayer and the gate electrode; forming a second insulating filmcomprising an organic material over the first insulating film; forming apixel electrode over the second insulating film; and forming anelectrode over the second insulating film and a portion of the pixelelectrode, wherein the electrode is directly connected to thesemiconductor layer through a contact hole opened in the gate insulatingfilm, the first insulating film and the second insulating film.
 2. Amethod for manufacturing a semiconductor device according to claim 1,wherein the gate insulating film comprises silicon oxide.
 3. A methodfor manufacturing a semiconductor device according to claim 1, whereinthe organic material comprises polyimide.
 4. A method for manufacturinga semiconductor device according to claim 1, wherein the pixel electrodecomprises ITO.
 5. A method for manufacturing a semiconductor deviceaccording to claim 1, wherein the electrode comprises a multi-layeredfilm comprising titanium nitride and aluminum.
 6. A method formanufacturing a semiconductor device comprising: forming a semiconductorlayer over a substrate; forming a gate insulating film over thesemiconductor layer; forming a gate electrode over the gate insulatingfilm; forming a first insulating film comprising an inorganic materialover the semiconductor layer and the gate electrode; forming a secondinsulating film comprising an organic material over the first insulatingfilm; forming a pixel electrode over the second insulating film; andforming an electrode over the second insulating film and a portion ofthe pixel electrode, wherein the electrode is directly connected to thesemiconductor layer through a contact hole opened in the pate insulatingfilm, the first insulating film and the second insulating film.
 7. Amethod for manufacturing a semiconductor device according to claim 6,wherein the gate insulating film comprises silicon oxide.
 8. A methodfor manufacturing a semiconductor device according to claim 6, whereinthe inorganic material comprises silicon oxide.
 9. A method formanufacturing a semiconductor device according to claim 6, wherein theorganic material comprises polyimide.
 10. A method for manufacturing asemiconductor device according to claim 6, wherein the pixel electrodecomprises ITO.
 11. A method for manufacturing a semiconductor deviceaccording to claim 6, wherein the electrode comprises a multi-layeredfilm comprising titanium nitride and aluminum.
 12. A method formanufacturing a semiconductor device comprising: forming a semiconductorlayer over a substrate; forming a gate insulating film over thesemiconductor layer; forming a gate electrode over the gate insulatingfilm; forming a first insulating film over the semiconductor layer andthe gate electrode; forming a second insulating film comprising anorganic material over the first insulating film; forming a pixelelectrode over the second insulating film; and forming an electrode overthe second insulating film after forming the pixel electrode, whereinthe electrode is directly connected to the semiconductor layer through acontact hole opened in the gate insulating film, the first insulatingfilm and the second insulating film.
 13. A method for manufacturing asemiconductor device according to claim 12, wherein the gate insulatingfilm comprises silicon oxide.
 14. A method for manufacturing asemiconductor device according to claim 12, wherein the organic materialcomprises polyimide.
 15. A method for manufacturing a semiconductordevice according to claim 12, wherein the pixel electrode comprises ITO.16. A method for manufacturing a semiconductor device according to claim12, wherein the electrode comprises a multi-layered film comprisingtitanium nitride and aluminum.
 17. A method for manufacturing asemiconductor device comprising: forming a semiconductor layer over asubstrate; forming a gate insulating film over the semiconductor layer;forming a gate electrode over the gate insulating film; forming a firstinsulating film comprising an inorganic material over the semiconductorlayer and the gate electrode; forming a second insulating filmcomprising an organic material over the first insulating film; forming apixel electrode over the second insulating film; and forming anelectrode over the second insulating film after forming the pixelelectrode, wherein the electrode is directly connected to thesemiconductor layer through a contact hole opened in the gate insulatingfilm, the first insulating film and the second insulating film.
 18. Amethod for manufacturing a semiconductor device according to claim 17,wherein the gate insulating film comprises silicon oxide.
 19. A methodfor manufacturing a semiconductor device according to claim 17, whereinthe inorganic material comprises silicon oxide.
 20. A method formanufacturing a semiconductor device according to claim 17, wherein theorganic material comprises polyimide.
 21. A method for manufacturing asemiconductor device according to claim 17, wherein the pixel electrodecomprises ITO.
 22. A method for manufacturing a semiconductor deviceaccording to claim 17, wherein the electrode comprises a multi-layeredfilm comprising titanium nitride and aluminum.
 23. A method formanufacturing a semiconductor device comprising: forming a semiconductorlayer over a substrate; forming a gate insulating film over thesemiconductor layer; forming a gate electrode over the gate insulatingfilm; forming a first insulating film over the semiconductor layer andthe gate electrode; forming a second insulating film comprising anorganic material over the first insulating film; forming a pixelelectrode over the second insulating film; and forming a pair ofelectrodes over the second insulating film, wherein the pair ofelectrodes are directly connected to the semiconductor layer through acontact hole opened in the gate insulating film, the first insulatingfilm and the second insulating film, and wherein one of the pair ofelectrodes is electrically connected to the pixel electrode.
 24. Amethod for manufacturing a semiconductor device according to claim 23,wherein the gate insulating film comprises silicon oxide.
 25. A methodfor manufacturing a semiconductor device according to claim 23, whereinthe organic material comprises polyimide.
 26. A method for manufacturinga semiconductor device according to claim 23, wherein the pixelelectrode comprises ITO.
 27. A method for manufacturing a semiconductordevice according to claim 23, wherein the electrode comprises amulti-layered film comprising titanium nitride and aluminum.
 28. Amethod for manufacturing a semiconductor device comprising: forming asemiconductor layer over a substrate; forming a gate insulating filmover the semiconductor layer; forming a gate electrode over the gateinsulating film; forming a first insulating film comprising an inorganicmaterial over the semiconductor layer and the gate electrode; forming asecond insulating film comprising an organic material over the firstinsulating film; forming a pixel electrode over the second insulatingfilm; and forming a pair of electrodes over the second insulating film,wherein the pair of electrodes are directly connected to thesemiconductor layer through a contact hole opened in the gate insulatingfilm, the first insulating film and the second insulating film, andwherein one of the pair of electrodes is electrically connected to thepixel electrode.
 29. A method for manufacturing a semiconductor deviceaccording to claim 28, wherein the gate insulating film comprisessilicon oxide.
 30. A method for manufacturing a semiconductor deviceaccording to claim 28, wherein the inorganic material comprises siliconoxide.
 31. A method for manufacturing a semiconductor device accordingto claim 28, wherein the organic material comprises polyimide.
 32. Amethod for manufacturing a semiconductor device according to claim 28,wherein the pixel electrode comprises ITO.
 33. A method formanufacturing a semiconductor device according to claim 28, wherein theelectrode comprises a multi-layered film comprising titanium nitride andaluminum.